In fuel cell systems installed in electric cars and the like, to respond to load fluctuations that cannot be followed due to the responsiveness of the fuel cell system, a hybrid system connected to the output terminal of the fuel cell that raises or lowers the voltage of the battery output is sometimes used.
In this type of hybrid fuel cell system, technology that takes into consideration the operating efficiency has been disclosed in for example Japanese Patent Application Laid-open No. 2002-118979, in which the ratio of the maximum output of battery and fuel cell is set to the range where the fuel cell is 65˜80% of the total output, and losses in a DC-DC converter are suppressed.